1. Technical Field
The present disclosure relates to a structure for growth of a nitride semiconductor layer, a multilayer structure including that structure, a nitride-based semiconductor device including that multilayer structure, and a light source including that nitride-based semiconductor device, and manufacturing methods thereof.
2. Description of the Related Art
A nitride semiconductor containing nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (GaN-based semiconductors) have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and semiconductor laser diodes in which GaN-based semiconductors are used as materials have already been used in actual products (see Japanese Laid-Open Patent Publications No. 2001-308462 and No. 2003-332697, for example).
The GaN-based semiconductor includes an AlxGayInzN (0≦x<1, 0≦z<1, 0<y≦1, x+y+z=1) semiconductor and has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (0≦x<1, 0≦z<1, 0<y≦1, x+y+z=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to as “C-axis” and “C-plane”.
In fabricating a semiconductor device using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which principal surface is a (0001) plane, is used as a substrate on which GaN semiconductor crystals will be grown. In a c-plane, however, there is a slight shift in the c-axis direction between a Ga atom layer and a nitrogen atom layer, thus producing electrical polarization there. That is why the c-plane is also called a “polar plane”. As a result of the electrical polarization, an internal electric field is generated due to spontaneous electrical polarization or piezoelectric polarization along the c-axis direction in the InGaN quantum well in the active layer. Once such an internal electric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer due to the quantum confinement Stark effect of carriers. Consequently, the internal quantum efficiency decreases. Thus, in the case of a semiconductor laser diode, the threshold current increases. In the case of an LED, the power dissipation increases, and the luminous efficacy decreases. Meanwhile, as the density of injected carriers increases, the internal electric field is screened, thus varying the emission wavelength, too.
Thus, to overcome these problems, it has been proposed that a substrate of which the principal surface is a non-polar plane such as a (1-100) plane that is perpendicular to the [1-100] direction and that is called an “m-plane” be used. As used herein, “-” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). As shown in FIG. 2, the m-plane is parallel to the c-axis (primitive vector c) and intersects with the c-plane at right angles. On the m-plane, Ga atoms and nitrogen atoms are on the same atomic-plane. For that reason, no electrical polarization will be produced perpendicularly to the m-plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m-plane, no piezoelectric field will be generated in the active layer, thus overcoming the problems described above. The “m-plane” is a generic term that collectively refers to a family of planes including (1-100), (-1010), (10-10), (-1100), (01-10) and (0-110) planes. As used herein, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c, m, etc.) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as “principal surface” or “growing plane”. A layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
Thus, for example, an LED which has such a non-polar plane as the principal surface can have improved emission efficiency as compared with a conventional device which is manufactured on a c-plane.
As of now, LEDs and laser diodes that employ a nitride semiconductor structure of which principal surface is an m-plane that is a non-polar plane have been realized in laboratories. Almost all of these laboratory studies employ, as the substrate for growth, a GaN bulk substrate of which principal surface is the m-plane. Therefore, the problems of lattice mismatch and thermal expansion coefficient difference between a growing film and the substrate would not occur, so that growth of a nitride semiconductor device structure with high crystal quality is possible, and a high efficiency LED and laser oscillation are realized.
However, this GaN bulk substrate which is presently used for crystal growth of a nitride-based semiconductor device of which principal surface is the m-plane is expensive as compared with a sapphire substrate which has been conventionally used for a c-plane GaN-based LED, and also, it is difficult to realize a large diameter.
For example, in the current market, the price of a GaN bulk substrate is higher than that of a sapphire substrate of the same size by two orders of magnitude or more. As for its size, a substrate of m-plane GaN has a square size of about 1-2 cm on each side, and even in the case of a bulk substrate of c-plane GaN, a large diameter of two inches or more is difficult to realize as of now. On the other hand, the sapphire substrate is presently inexpensive, e.g., several thousands of Japanese yen, so long as its size is two inches, large diameters of four inches, six inches, and greater, have already been realized.
Thus, it can be said that, even in nitride semiconductor growth on an m-plane that is a non-polar plane, using sapphire as the substrate is particularly advantageous from the viewpoint of cost reduction.
On a sapphire substrate of which principal surface is the m-plane (hereinafter, “m-plane sapphire”), an m-plane nitride semiconductor can be grown (PCT INTERNATIONAL APPLICATION PUBLICATION NO. 2008/047907). Further, on the m-plane sapphire substrate, (11-22) plane and (10-1-3) plane, which are semi-polar planes, can be grown under predetermined conditions (Japanese Journal of Applied Physics 45, No. 6, L154-L157 (2006)).
In general, a nitride semiconductor crystal grown on a heterogeneous substrate which has a different crystalline structure, lattice constant, or thermal expansion coefficient, such as a sapphire substrate, (i.e., a so-called “hetero-grown” nitride semiconductor crystal) includes threading dislocations with high density (that mean edge dislocations, screw dislocations, and mixed dislocations, which are generically and simply referred to as “dislocations”) and stacking faults. This is mainly attributed to a large lattice mismatch degree and a difference in crystalline structure between the nitride semiconductor and the different type of substrate. Dislocations and defects caused at the interface between the different type of substrate and the nitride semiconductor reach the active layer or device surface, significantly deteriorating the device characteristics, such as decrease in efficiency of the LED, decrease in device life. Since stacking faults are usually produced in the c-plane, in a nitride-based semiconductor device grown on a conventional c-plane sapphire, the stacking faults would not extend in the growing direction. Therefore, in the conventional c-plane growth, stacking faults do not reach the active layer. However, in growth of a non-polar plane, there is a c-plane lateral surface, and therefore, there is a probability that stacking faults produced in the c-plane reach the active layer or device surface, and this can be a major cause of deterioration in device characteristics. Thus, in a nitride-based semiconductor device of which principal surface is a non-polar plane, in order to realize a highly-efficient LED or laser diode, it is necessary to reduce the stacking fault density in addition to the threading dislocation density.
One known technique for reducing these dislocations and stacking faults is a selective growth method in which a mask pattern is employed. Such a growth method is commonly referred to as “Epitaxy Lateral Over Growth (ELOG)”.
Epitaxial lateral overgrowth with the use of a semi-polar or non-polar GaN is already reported in Japanese Laid-Open Patent Publication No. 2009-295994 and other documents. In Japanese Laid-Open Patent Publication No. 2009-295994, a GaN film of which principal surface is an A-plane is grown on an R-plane sapphire substrate, and a SiO2 mask is formed on that GaN surface for achieving epitaxial lateral overgrowth. In Japanese Laid-Open Patent Publication No. 2009-295994, the plane orientation dependence of the lateral growth was examined with varying stripe-shaped SiO2 mask orientations in the plane.
One of the other epitaxial lateral overgrowth methods is a Pendeo growth method. This method is similar to the previously-described ELOG method but different in that lateral growth is carried out using a substrate which has an uneven structure. Since regrowth of a nitride semiconductor starts only from raised portions of the uneven structure, a regrown film is in a hung state, which is why the method is named Pendeo (“hang” in Latin).
There are some similar epitaxial lateral overgrowth methods, and also, there are some similar names for the growth methods. In this specification, methods wherein a substrate that has an uneven structure including nitride semiconductor regions which serve as growth cores or starting points for regrowth and heterogeneous substrate surface regions which are exposed by processing, such as etching, is provided, and a nitride semiconductor film is selectively regrown from the nitride semiconductor regions of the raised portions of the uneven structure, are generically referred to as “Pendeo growth”. A manner of Pendeo growth in which the regrowth is carried out with the mask remaining at the crest portions of the raised portions is simply referred to as “Pendeo growth” or “masked Pendeo growth”, whereas a manner of Pendeo growth in which the regrowth is carried out without the mask at the crest portions of the raised portions is referred to as “maskless Pendeo growth”.
There are also lateral selective methods in which, in Pendeo growth, the surface of the recessed portions is covered with a dielectric (Applied Physics Letters 76, 3768 (2000) and Applied Physics Letters 75, 2062 (1999)). These methods are commonly called “air-bridged ELO” or “LOFT (lateral overgrowth from trenches)”. In the air-bridged ELO, etching is not continued till the heterogeneous substrate is exposed. Rather, the nitride semiconductor film is etched to some extent, and thereafter, that nitride semiconductor surface is masked with a dielectric material. In the LOFT, the surface of the heterogeneous substrate which has been exposed by etching is masked with a dielectric material. These methods would not cause regrowth on the dielectric mask as in the previously-described ELOG method.